The invention relates generally to vehicle transmission control devices and more specifically to a combined control device for a main transmission and auxiliary two-speed auxiliary range box.
Manual transmissions having two or three shift rails which provide reverse as well as three or five forward gear ratios, respectively, provide appropriate and sufficient gear ratio ranges and forward speeds to fulfill the requirements of a vast majority of transmission applications. Transmissions providing greater numbers of gear ratios, however, are routinely produced but their size, complexity and cost is representative of their specialization and rather more limited application.
Devices do exist, however, which extend the gear ratio range and thus capabilities of, for example, transmissions having five forward gears. Such devices are auxiliary gear boxes which are operably disposed in tandem with the main transmission. Typically, such auxiliary gear boxes or range boxes provide two gear ranges generally designated high and low which, when combined with the several ranges of the main transmission, effectively doubles the number of gear ranges which the overall transmission assembly provides. In practice, the plural ranges provided by the transmission when the auxiliary gear box is in one range typically overlap those provided when the auxiliary transmission is in its higher range. Thus, in practice, the auxiliary gear box is generally utilized only to augment the higher gears of the transmission by providing still lower overall gear ratios through the transmission assembly and thus improve fuel economy and highway performance.
A common means of permitting operator selection of the range of the auxiliary gear box comprehends providing a two position switch which may be controlled by the vehicle operator to select the desired range. Frequently, such switch is secured directly to the shift lever such that the operator may select both the gear of the main transmission and the range of the auxiliary transmission with one hand. Such a configuration, however, permits free choice of the range of both the main transmission and auxiliary gear box thus typically providing several ranges which overlap. While this is not specifically undesirable, such a situation tends to promote unnecessary gear shifts, clutch wear and less than optimum fuel economy and performance. This is to say nothing of the inconvenience created by the complex motion required to move both the shift lever and the range selector switch attached thereto. Another difficulty of such independent control over the main and auxiliary gear boxes relates to the possibility of inadvertently effecting a change in the ratio of the auxiliary gear box while the clutch is engaged and power is being transmitted through the transmission assembly. Since such a shift is undesirable, it is considered to be beneficial to include means whereby a change between the ratios of the auxiliary gear box is prohibited unless the main transmission is in neutral and thus not transferring power.